1. Field of the Invention
The present invention relates to a sensing membrane on an ion sensitive field effect transistor (ISFET), and in particular to a method for fabricating a titanium nitride (TiN) sensing membrane on an extended gate field effect transistor (EGFET). The method forms the TiN sensing membrane as an ion sensitive sensor (pH sensor) using a radio frequency (RF) sputtering process and a standard complementary metal oxide semiconductor (CMOS) process. The method uses the standard CMOS process for mass production providing the benefits of low cost, high yield, and high performance.
2. Description of the Related Art
A glass electrode has typically been used as an ion sensing electrode. A glass electrode has the disadvantages of difficulty in measuring miniaturized structures, susceptibility to damage, and low portability. To solve those problems, Piet Bergveld introduced an ion sensitive field effect transistor (ISFET) in 1970. ISFET has the advantages of compatibility with standard CMOS process, high input impedance, and low output impedance, miniaturization capability minute solution measurement, rapid response, and pH measurement capability. Therefore, ISFET has been developed and applied in a variety of fields relating to miniaturized pH sensors and biotechnical sensing devices.
In the 1970s, research on ISFET pH sensors and related applications had just begun. In the 1980s, research on ISFET pH sensors had progressed to a more advanced level regarding aspects of fundamental theories, crucial techniques, and practical applications. Over 30 different ISFETs have been introduced to measure several kinds of ion and chemical materials. Great strides have also been made in miniaturization, modulization, and multi-functionality. According to current research, a hydrogen ion sensing membrane on a gate oxide layer of an ISFET typically comprises silicon dioxide (SiO2), silicon nitride (Si3N4), tantalum oxide (Ta2O5), aluminum oxide (Al2O3), or other materials. These materials have the disadvantages of low linear sensitivity and low photostability, particularly after long-term operation. Additional processes are also required to handle these materials. Due to the described problems, a replacement material is called for.